A special line is employed as a trunk line for a conventional time division multiplexer (TDM). For transmission of speech, a countermeasure is taken whereby compression of speech is performed to reduce the width of the bands in a trunk line, so that the number of lines contained increases. Various speech compression methods are available and are selected by taking compression rate and speech quality into consideration. Currently, the following speech methods are considered methods for selection. PCM transfer in which compression is not performed is also listed.
______________________________________ Compression Communication Compression Compression Speech Method Speed Rate Effect Scale Quality ______________________________________ 1. PCM 64 Kb/s 1 1.0 1.0 2. ADPCM 32 Kb/s 2 0.5 0.8 3. LDCELP 16 Kb/s 4 0.25 0.7 4. VSELP 8 Kb/s 8 0.125 0.6 ______________________________________
In the background, however, since a method is used whereby a speech compression system is selected each time a call occurs, the following problems will arise:
1. When a speech line is to be set, the empty capacity of a trunk is taken into consideration and an optimal compression system is selected to ensure a necessary band. PA0 2. If there is no empty space in the trunk, a line can not be acquired relative to a request for a call, and a call loss occurs. PA0 3. When a line is used for only a high compression system, speech quality is always poor and does not satisfy a user.
Disclosed in Japanese Patent Application No. Sho 62-160899 is a trunk line selection method by which, in a time-division speech compression system that can select a plurality of speech compression methods, when a trunk line for selected speech quality (compression system) is being used, a trunk line for a higher speech quality (compression system) is employed. This trunk line selection method is, however, a method for selecting one of a plurality of compressed speech qualities relative to a generated call, and as the currently connected calls are dynamically re-connected, it is impossible to effectively utilize the band of a line by switching the selected compressed speech quality, and to avoid a call loss relative to a new call. When there is no empty space in the trunk line, a line band can not be ensured for a dialing request, and a call loss will occur.
Disclosed in Japanese Patent Application No. Sho 62-143531 are a compound communication system, which includes speech information, data information and frame information, that sets a compression rate for speech information so that the communication capacities of the speech information, the data information, and the frame information are the maximum channel capacities. However, this background does not teach the dynamic re-connection of a currently connected call, and when all the trunk lines are being used by the compression system of a higher speech quality, it is impossible to switch the compressed speech quality for effective use of the band width of a line and to avoid a call loss relative to a new call.